Device for maintaining a fixed bias



May l5, 1945- o. THOMPSON 2,375,877

DEVICE FOR MAINTAINING A FIXED BIAS Filed Nov. i4, .1942

Patented May 15, 1945 rville I. Thompson,

Ille Forests.. Training poration ofi' Illinois4 Chicagm. I1I. assgnon toA Inc., Chicago.: Ill.,I a cor- Applicationf-1\ovemlier 14, 1942, Serial No. 465,595

(CI. Z50- 27.)v

` Claims.

This inventionrela-tes to electron discharge-devices andA more-particularlyfto a' devi'ce for maintaining a XedL bias from an alreadyv existing power supply in thermionicV tube circuits.

It is one of theprincipal"obje'cts'of thisinvention to provide a deviceformaintaining aA fixed biasy in electronic discharge circuits which will be simple in construction, economical in manufacture-andA highlyefcient in use.

Another object of*1 this` invention is to provide in; electron circuitsv a fixed; negative potential having' a; source independent off the signal cir'- cuit andV dependent" upon a uniform voltage supplynormallyy required in the operation of certain other elements employed' in the electronA dis'- charge circuit.

A- further object of1 thisinvention is to provide in radio circuits employing a; multi-elementtube, a rectierv circuit in. the tube for converting already existing alternating, current into a uniform direct current to supply ay negative potential#I to a grid circuit also employed in the tube.

Other objects will appear hereinafter;

The invention consistsv in the` novell combination and arrangement of parts'tok be hereinafter described andclaimed.

The invention will be best understood by reference to the accompanying` drawing in which:

Fig; l is a wiring diagram of` an audio circuit embodying my invention;` i

Fig. 2 is a wiringI diagram of another formpf circuit embodying my invention; and

Fig. 3 is. only that portion ofV a wiring diagram requiredvto carry out the essence of my. invention.

In' the obtaining of maximum. operating efci'encyfin eitherv oscillator or amplifier circuits in which electron discharge devices are employed, it is essential that the various electrodes of such electron discharge devices be' maintained; at an electrical potential which varies only in proportion to the input or signal voltages. In maintaining the various electrodes' at the desiredele'ctrical potential, the grid element is often operated at a certain fixed negativel potential' with respect to the anode and cathode elements of the' tube, which difference of potential between the gridv element and' anode and cathode elements is calledthe bias voltage.

Various circuit arrangements have been employed for Supplying ai fixed bias voltage to radio circuits. One common method of: applying thenegativeV grid. potentialV is toy employ a separate sourceofy electrical energy suchas. adrycell and: while this methodaccomplishes. the result ofA supplying a bias voltage, it has not been'found satisfactory becausey such dry cells require additional space, are expensive in installation and due to their'high mortalityrate must be replaced toofrequently. Separate recti'er units have been employed' torectify'A. C. power supply to producea fixed bias voltage forobtaining the optimum operating eiiiciency ofthe electron tube, but the size of such units and the additional expense of construction and material of' separate rectiers are prohibitive in the production of most electronic devices, and therefore such devices are not satisfactory.

Other methods of obtaining a negative poterl-l tial for the grid circuit have been the semi-fixed bias method and the self-bias method, both of. rwhich are not entirely independent of the signa-l'- circuit and; thus are variable and not fixed and. will fluctuate with the signal, thus developing a distortion inthe plate circuit.

In the presentinventiomthe usual circuitis em,-

ployedv but in additionthereto a xedbias voltage is produced; from a uniform voltage supply al'- ready employed in the circuit for another purpose and which voltage supply is independent from the signalv circuit. In the most desirable method of carrying out my invention, reference is made to the drawing. where in Fig. 1 there is shown. an. amplier circuit generally indicated at l0.. In this circuit is arranged an electronic discharge device or tube I I and this tube contains the usu.- al'grid l2,` cathode I3 vand plateor anode I4. 'I'he cathode I3 is indirectly heated by a lament I5 which obtains a low voltage. supply of current by way of conductors 311-311' from'a, heater supply in the form ofthe secondary winding IS of a..

transformer Il having thev usual primary winding` I.8 connected to an A. C. power supply i9.

As shown in Fig. 1, there is' provided an inputl transformer 20. which is connected to the grid' circuit ZIV whichincludes a. conductor 22e`xtendvl ing from the transformer 2lil through a grid condenser 23 to the grid I2, and a conductor 2d extending from the cathode I3' Iback to the 0pposite terminal ofthe input transformer 2D'. An

output transformer 25 is arranged in the circuit lland isconnected by conductor 2B to the plate I t to transmit plate current to the primary winding of the output transformer 25, the opposite terminal of the output transformer primary being connected via conductor 2l' to the positive side of" the plate supply voltage which is direct current or rectified` A. C. from the secondary winding 28? of the transformer lily', the negative side of said plate voltage supply being connected via conductor 29 to the cathode I3 via conductor 24.

A grid load in the form of a resistance 3I is connected across the leads 22 and 24 of the grid circuit 2| and to this is connected the grid bias voltage supply which is the essence of my invention. For this purpose the tube II is a multi-element special purpose tube of any of the conventional and well-known types which include, in addition to the electrodes already mentioned, one or more dio-de plates 32. These diode plates 32 in conjunction with the cathode I3 have a rectifying action on current bridging the diode and cathode and therefore by connecting these two electrodes to a xed source of low voltage A. C. a fixed D. C. voltage will be produced, which D. C. voltage is adapted for use as a iixed bias voltage.

'I'he voltage supply to the heater element I5 being of a low voltage and being otherwise uniform and unrelated to the signal circuit, has been found to be an ideal source of low voltage alternating current. I therefore connect the conductor 30 to the diode plate 32 and the conductor 30' to the cathode I3 and, in addition, I provide a filter for the direct current, thus provided, by connecting aresistor 33 from the conductor 24 leading from the cathode I3, and connect the opposite terminal of the resistor 33 to the positive conductor 30 (with respect to thediode circuit) of the heater Asupply circuit by way of conductor 34. The iilter further includes a shunt condenser 35 which is connected in parallel with the resistor 33 and together with the latter is connected in series with the grid resistance 3l.

The wiring diagram illustrated in Fig. 2 is another form of circuit embodying the identical form of bias circuit but employing only a single diode electrode. The reference characters of Fig. 2 are the same as those applied to Fig. l, with the exception of the separate transformer unit I1' and its primary winding I 8' for the heater voltage supply and the inclusion of a plate resistor 36 in the output or the plate circuit.

As shown in Fig. 3, the heater element I will glow to produce a continuous heat, therelby indirectly heating the cathode I3 to cause electronic emissions from the cathode I3 to the diode plate 32, thus permitting part of the alternating current to flow through the diode rectifier elements of the tube I I. The rectifying action of the diode plates 32 will cause the effective portion ofthe alternatingcurrent to appear as pulsating direct current across the resistor 33, which with the shunt condenser 35 will provide a uniform D. C. voltage. The grid circuit 2| being shunted across the resistor 33 through the grid leak resistance 3|, the voltage across the resistor 33 will be appilied to the'grid circuit as a iixed negative potent al.

The current in the plate supply line passes through the cathode directly to the plate supply negative and has no eiect on the voltage developed across the resistor 33. It will therefore be seen that the bias voltage produced by my invention is entirely independent of the signal voltage with respect to both input and output sides of the tube circuit, and for that reason a iixed bias is obtained. Y

While I have illustrated and described the preferred form of construction for carrying my invention into eifect, this is capable oflvariation and modification without departing from the spirit of the invention. I. therefore, do not wish to be limited to the precise details of construc- F tion set forth, but desire to avail myself of such variations and modifications as come within the scope of the appended claims.

Having thus described my invention, what I claim as new and desire to protect by Letters Patent is:

l. A bias circuit for an electronic device of known type Which includes a plurality of anodes, a cathode for said anodes, a heater for said cathode and having a pair of terminals adapted to be connected to a source of alternating current, a control grid between a first one of said anodes and said cathodesaid Ibias circuit comprising: a bias resistor having a first terminal connected to a first terminal of said heater, said bias resistor having a second terminal connected to said cathode, a second anode connected to a second heater terminal aforesaid whereby to produce a uni-directional voltage drop across said bias resistor, means connected with said bias resistor for smoothing the bias voltage developed thereacross, a leakage resistance connecting said grid with the negative terminal of said bias resistor, and an input circuit connecting with said grid and said cathode, whereby to actuate said device without substantially loading said bias circuit.

2. In a bias circuit for an electronic tube of the type which includes an anode, a diode plate, a control grid, a cathode adapted to be energized from a source of alternating current, bias means comprising: a resistor connected between said cathode and one energizing terminal for said heater, a second resistor connected between said control grid and that terminal of the first-mentioned resistor which is connected to the heater aforesaid, filter means shunted across said nrstmentioned resistor, and a connection between a second terminal for said heater and said diode plate, whereby said grid is rendered negative with respect to said cathode, together with input connections to said grid and cathode, adapted to impulse said grid without traversing said first-mentiond resistor.

3. An electronic tube circuit comprising: a tube having a grid, a main anode, a rectifier anode, a

cathode for said anodes, a heater filament for said cathode, said heater filament having a pair of terminals adapted for connection to a source of alternating current, a pair of resistors having a common bias terminal connection with one of said pair of heater terminals, a first one of said resistors having a remaining terminal connected to said cathode, the other said resistor having a remaining terminal connected to said control grid, a filter reactance shunted across the terminals of said iirst resistor, and a load circuit shunted across the aforesaid remaining terminals of the respective resistors, said first resistor providing a negative Ibias potential for said grid. whereby no effective portion of energy from said load traverses said first resistor to disturb the bias potential provided thereby. r

4. A bias circuit for an electronic tube of the type which includes a first anode, a control grid for said anode, a rectifier anode, a cathode for said anodes, a heater for said cathode and having energizing terminals adapted to be connected to a source of heater energy, said circuit comprising: a resistance component connected in shunt across said control grid and said cathode, a bias connection to said resistance component at a point between the aforesaid shunt-connection terminals therefor, said bias connection beining connected to an energizing terminal for said heater, a connection between said rectiiler anode and another said energizing terminal for the heater, a filter condenser shunted between said Joias connection and said cathode, said tube being adapted to have a signal load connected across said resistance component, said Ibias connection providing a negative bias potential for said control grid which bias potential is substantially unaiected by said load.

5. Apparatus of the class described comprising a thermionic tube including a cathode, a heater for said cathode and adapted to Iloe energized from a source of alternating current, a rst anode cooperable with said cathode, a grid for controlling space current between said cathode and anode, a second anode cooperable with said cathode, circuit connections applying the alternating potential across said heater to said second anode and said cathode, whereb'y to provide a rectified bias potential, reactance means connected to smooth said bias potential, and connection applying the negative aspect of said bias potential to said grid, whereby to render the latter negative with respect to said cathode.

- ORVILLE I. THOMPSON. 

